METHOD FOR PRODUCING MINIMUM-RIDGING TYPE 430 Mo STAINLESS STEEL SHEET AND STRIP

ABSTRACT

A process for producing a Type 430 Mo stainless steel sheet product with substantially reduced ridging characteristics when drawn. Commencing in slab form, the Type 430 Mo steel is hot rolled at 2000*-2100*F and finished at below 1400*F, with a reduction of at least 30 percent per pass; box annealed at 17501850*F for at least 10 hours and then at 1450*-1500*F for at least 2 hours; cold rolled to final thickness; and continuous annealed at 1575*-1625*F.

0 United States Patent v[151 3,655,459 Brickner et al. [451 Apr. 11, 1972 METHOD FOR PRODUCING MINIMUM- [56] References Cited RIDGING TYPE 430 M0 STAINLESS UNITED STATES PATENTS STEEL SHEET AND STRIP 3,139,358 6/1964 Graziano ..l48/l2 [72] inventors: Kenneth G. Brickner, OHara Township, 320L231 8/1965 Hamster? Allegheny County; George Bethe Pn'mary Examiner-L. Dewayne Rutledge Park, both of Pa. Assistant ExaminerW. W. Stallard [73] Assignee: United States Steel Corporation Attorney-Forestasexton [22] Filed: Aug. 13, 1970 [57] ABSTRACT [21] Appl. No.: 63,612 A processfor producing a Type 430 Mo stainless steel sheet product with substantially reduced ridging characteristics when drawn. Commencing in slab form, the Type 430 Mo [52] U.S.(l ..148/12 steel is hot rolled at 20002100F and finished at below 5: 1m. CL ..C21d 9/48 1400F, with a reduction of at least 30 p n p p ox 581 Field oiSearch ..14s/12 annealed at 1750-1850": for least 10 hours and at 1450-1500F for at least 2 hours; cold rolled to final thickness; and continuous annealed at l575- l 625F.

4 Claims, No Drawings METHOD FOR PRODUCING MINIMUM-RIDGING TYPE 430 MOSTAINIESSSTEELSHEE'IANDSTRIP BACKGROUND OF THE INVENTION It is well known that the 17 percent chromium ferritic stainless sheet steels (i.e., AlSl Type 430 stainless steels) usually exhibit a surface defect known as ridging or "roping" when subjected to severe cold forming operations such as deep drawing. This defect appears as narrow raised areas similar to corrugations parallel to the cold rolling direction. Ridging is of course detrimental to the appearance of the drawn part, and expensive grinding and polishing operations are required to suitably improve the appearance.

Although many processes and procedures have been developed to overcome the ridging problems associated with the AIS] Type 430 stainless steels, these processes have not been altogether satisfactory, at least from a commercial point of view. According to one process for example (U.S. Pat. No. 2,965,479), ridging can be eliminated by adding small amounts of columbium to the alloy. Although this process is used commercially to a limited extent, it greatly adds to the cost of the product, not only because of the cost of the columbium itself, but because the columbium causes processing difficulties resulting in lower yields. Most other procedures for eliminating the ridging problem have involved the careful control of processing parameters, i.e., rolling and annealing, to effect particular microstructures allegedly found to be nonridging in nature. Of particular significance to this invention is a process disclosed in U.S. Pat. No. 2,851,384, Waxweiler, Sept. 9, 1958. According to this patent, ridging is minimized by increasing the amount of austenite to above 35 percent at the hot rolling temperatures, and then subjecting the steel to carefully controlled heat treatments to randomize the texture or orientation of the hot rolled grains. The austenite content is controlled through close control of carbon, nitrogen, silicon, nickel and manganese in a 14-18 percent chromium stainless steel. Although this process, and other prior art processes, are effective to some extent in minimizing ridging in conventional Type 430 stainless steels, none of the prior art processes have been effective in reducing ridging in the molybdenum containing Type 430 Mo stainless steels, which is particularly desirable for many drawn products because of its enhanced resistance to corrosion. On the contrary, ridging is particularly troublesome and severe in Type 430 Mo stainless steels. Therefore, despite its commercial appeal due to greater corrosion resistance, no method is known whereby ridging in Type 430 Mo stainless steel sheet can be eliminated or minimized.

SUMMARY OF THE INVENTION An object of this invention is to provide a Type 430 Mo stainless steel sheet which can be drawn with little or no ridging in the strained portions thereof.

Another object of this invention is to provide a process for producing a Type 430 Mo stainless steel sheet having substantially reduced ridging characteristics when subjected to severe cold forming operations, and further having tensile properties equivalent to those of prior art Type 430 Mo stainless steel sheets.

These and other objects and advantages will become apparent from the following detailed description of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted above, the process of this invention is applicable only to the molybdenum-modified Type 430 Mo stainless steel, having a nominal composition as follows:

carbon 0.15% maximum manganese 1.0% maximum silicon 1.0% maximum chromium l6 to [8% molybdenum 0.75 to 2% iron balance Besides the above listed elements, the alloy may of course contain the usual steelmaking impurities in amounts insufficient to afiect the alloys properties.

Stated briefly, the process of this invention, whereby a Type 430 Mo stainless steel is produced with minimum ridging characteristics, involves the careful control of almost all processing parameters in producing the sheet from slab form. Of critical significance are the hot rolling temperatures and reductions, the box annealing times and temperatures following hot rolling, cold reductions, and the continuous annealing temperatures following cold reduction.

More specifically, the preferred process of this invention comprises (1) hot rolling a slab of the steel at a starting temperature between 2000 and 2l00F with a minimum reduction of 30 percent per pass; (2 finishing-the hot rolling operation at a temperature below 1400F to yield a hot band between 0.1 and 0.25 inch thick; (3) box annealing the hot band between l750 and l850F for at least 10 hours, after which the temperature of the annealing furnace is lowered to about l450-l500F for at least 2 hours; (4) pickling and then cold rolling the annealed hot band between 30 and 50%; and (5) continuous annealing the cold rolled sheet between l575 and 1625F and pickling to yield the final product. For thinner sheet materials, steps (4) and (5) can be repeated.

Although the details of the mierostructural development and the cross relationships between the various parameters are not completely understood, we have found that each of the above parameters is critical if a substantially non-ridging product is to be achieved.

For reasons completely unknown to us, the slab hot rolling temperatures and reductions are critical as noted. Although ridging characteristics can be reduced to some extent by commencing hot rolling within the range l900 to 2200F, optimum results are achieved at preferred starting temperatures of from 2000 to 2100"F. Similarly, appreciable reductions in ridging tendencies can be achieved only if the hot reductions are of about 30 percent or more.

The box annealing procedure following hot rolling must also be critically controlled if the ridging characteristics in the final product are to be reduced. Generally speaking, increases in annealing temperature, up to 1850F, and/or increases in annealing time, up to 12 hours, will cause some improvement in the products resistance to ridging. Such improvements, however, are quite small and insignificant unless the annealing temperature is within the range l750 to 1850F, and the annealing duration at least 12 hours. As will be noted below, however, the annealing temperature can and should be lowered for the final 2 hours of the anneal. Annealing durations of less than 12 hours are insuficient to yield a product having significant and useful resistance to ridging. On the other hand, periods in excess of 12 hours, up to 72 hours, have neither favorable nor adverse effects.

The dual temperature feature of the box anneal, as described above, is primarily to prevent an undesirable open" surface characteristic. That is to say, the pickling operation to remove surface scale following the box anneal has a tendency of producing an open or pitted surface. We believe this is caused by the pickling solution dissolving surface metal adjacent to precipitated carbides at the grain boundaries, which are formed during the high temperature box anneal. This detrimental open surface can be markedly reduced by lowering the temperature of the annealing furnace after a minimum of 10 hours at l750-l850F to the range l450- 1500"F and holding within this latter temperature range for at least 2 hours. This low temperature treatment reduces the susceptibility of the grain boundaries to attack by the pickling solution. If preferred, the metal can be completely cooled to ambient temperatures between the l750-l850F anneal and the l450- l 500F anneal without adverseefi'ect.

The cold rolling procedure following the box anneal should reduce the annealed hot band from about 30 to about 50 percent. Here again the reasons for these limits are not completely understood. Nevertheless, cold reductions of 30 to 50 percent have statistically been shown to produce a greater degree of resistance to ridging.

As stated above, the final continuous anneal should be at a temperature within the range 1S75 to 1625F. Temperatures below l575F down to about 1400F will not necessarily be seriously detrimental, but will not of course provide the best possibie product. On the other hand, temperatures modestly in excess of 1625F or 1700F may produce undesirable low elongations and/or coarse grain sizes. Hence, the upper limit of 1625F for the continuous anneal is somewhat more critical than the lower limit of 1575F.

In order to establish the above parameters, four different heats of steel were prepared having compositions as shown in the table below.

TABLE I.COMPOSITIONS OF TYPE 430 M STAINLESS STEELE INVESTIGATED, PERCENT From these four heats, over 100 test specimens were processed at differing parameters to ascertain the varying effects of the difiering parameters. The variables studied in detail were (1) slab rolling temperatures; (2) the amount of reduction during hot rolling; (3) box annealing temperatures; (4) box annealing times; (5 the amount of cold reduction; and (6) the continuous annealing temperatures following cold reduction.

For testing purposes, specimens 1% inch wide by 14 inches long were cut from the finished product in the longitudinal direction and then polished into a mirror finish. All specimens were then strained 15 percent in tension at room temperature. Specimens were visually examined and rated for severity of ridging according to the following arbitrary scale: 0 none; 1 very slight; 2 slight; 3 moderate; 4 severe; and 5 very severe.

Although the detailed data obtained from these tests is too voluminous to be presented here, the following examples will illustrate typical results.

For one series of tests, a commercial slab product of Steel 1 (see table) 5 inches thick was hot rolled at 2200F into 1 inch thick material. Specimens of the 1 inch slab were then hot rolled into 0.150 inch material at starting temperatures of 1900", 2000 or 2100F at varying amounts of reduction per pass, at percent or 40%. The hot rolled strip was given simulated box annealing at either 1600F or 1800F for 1 or 24 hours, and then furnace cooled. The annealed strip was then cold rolled to 0.07 inch sheet in 2 passes, annealed for 15 minutes at either l450 or 1600F, air cooled, cold rolled to 0.025 inch sheet in two passes and again annealed for 15 minutes at the same temperature, and finally air cooled. For testing purposes, the procedure outlined above was followed. Those specimens which were processed in accordance with this invention, i.e., hot rolled at 2100F, hot reduced by 40 percent, box annealed at 1800F for 24 hours and given continuous anneals at l600F, all had ridging ratings of 0.5. Those sessineaehsi s'lssll? Per ent "auctions had ridging ratings of from 1 to 4 with an average of 2.7 for 48 specimens. Those specimens box annealed at 1600F had an average ridging rating of 3.2, and so on.

In another series of tests, a commercial slab product of Steel 1 (see table) 5 inches thick was hot rolled at 2200F into 1 inch material. Specimens of this 1 inch slab were then hot rolled into 0.20 or 0.09 inch strip at either 2000 or 2200F. The hot rolled strips were given simulated box annealing at 1600" or l800F for 12, 24, 48 or 72 hours and furnace cooled. The 0.09 inch specimens were cold rolled to 0.04 inch sheet in 2 passes, and the 0.2 inch specimens were cold rolled to 0.09 inch sheet in 2 passes. All material was then heated for 15 minutes at 1450F and air cooled. The 0.04 inch material :was cold rolled to 0.025 inch in one pass (35 percent reduction), whereas the 0.09 inch material was cold rolled to 0.025 inch in two passes (70 percent reduction). Prior to testing, all 0.025 inch material was annealed for 15 minutes at 1450F and air cooled. All those specimens processed in accordance with this invention, i.e., hot rolled at 2000F, box annealed at l 8@F, and cold rolled at a 35 percent reduction exhibited ridging rati ngs of l Those specimens processed outside the scope of this invention exhibited an average ridging rating of 2 for 24 specimens. Those specimens with two or more amete QBL l ths wne Qflth s invention a in ratings of 2 to 3.

Similarly other tests were conducted to study the eflect of hot rolling finishing temperatures of 1400, l500, 1600", l700 and l800F which showed improved results as this variable temperature was reduced. Still further tests considered the variable efiects of cold reductions at 30 percent, 45 percent and 60 percent. Those specimens cold rolled at 30 and 45 percent had ridging ratings of O to 0.5 when the other processing parameters were within the inventive ranges.

We claim:

1. A process for producing a Type 430 Mo stainless steel sheet product with substantially reduced ridging characteristics when drawn comprising forming a steel slab having the composition by weight percent:

carbon up to 0.15%

manganese up to 1.0%

silicon up to 1.0%

chromium 16 to 18% molybdenum 0.75 to 2.0%

iron balance except for incidental impurities;

hot rolling said slab at a starting temperature within the range 2000 to 2 F, and a finishing temperature below 1400 F, with at least a 30 percent thickness reduction per pass; box annealing the hot rolled steel at a first annealing temperature of from 1750 to 1850 F for at least 10 hours and at a second annealing temperature of from l450 to l500 F for at least 2 hours; cold rolling the annealed steel to final thickness; continuous annealing the cold rolled steel at from l575 to l625 F; and cooling the steel product.

2. The process of claim I in which the cold rolling effects a thickness reduction of from 30 to 50 percent.

3. The process of claim 1 in which the cold rolling is performed in two steps with an intermediate continuous anneal of from 1575 to 1625 F.

4. A Type 430 Mo stainless steel sheet product having substantially reduced ridging characteristics when drawn, consisting of the steel produced in accordance with claim 1. 

2. The process of claim 1 in which the cold rolling effects a thickness reduction of from 30 to 50 percent.
 3. The process of claim 1 in which the cold rolling is performed in two steps with an intermediate continuous anneal of from 1575* to 1625* F.
 4. A Type 430 Mo stainless steel sheet product having substantially reduced ridging characteristics when drawn, consisting of the steel produced in accordance with claim
 1. 